This invention relates to an electrophotographic photosensitive plate, and more particularly to an improved multi-layered electrophotographic photosensitive plate comprising a photosensitizing layer of vitreous selenium and tellurium and a charge-retaining and charge-transporting top layer of an organic material.
It is known that images may be formed and developed on the surface of certain photoconductive insulating materials by electrostatic means. The basic electrophotographic process, as taught by Carlson in U.S. Pat. No. 2,297,691, involves uniformly charging a photoconductive insulating layer and then exposing the layer to a light-and-shadow image which dissipates the charge on the areas of the layer which are exposed to light. The electrostatic latent image formed on the layer corresponds to the configuration of the light-and-shadow image. This image is rendered visible by depositing on the image layer a finely divided developing material comprising an electroscopic marking material called a toner. This powder developing material will normally be attracted to those portions of the layer which retain a charge, thereby forming a powder image corresponding to the latent electrostatic image. This powder image may be transferred to paper or other receiving surfaces. The paper then will bear the powder image which may subsequently be made permanent by heating or other suitable fixing means. The above general process, the so called Carlson process, is also described in U.S. Pat. Nos. 2,357,809, 2,891,011 and 3,079,342.
In the electrophotographic process, it is desired that the electrophotographic photosensitive plate can be highly charge-retaining when subjected to the charging, that the charge retained on the plate can be quickly discharged to an electrically conductive substrate when the plate is exposed to a visible light be high with respect to a wide range of wavelengths of light (i.e. that it be is high with respect to wide wavelength range of the light (which panchromatic), and that the residual potential (charge) on the plate after the light exposure be low. For using the plate in a roll form or a bent form, it is desired that the plate be flexible.
French Pat. No. 72/36769 discloses an electrophotographic photosensitive plate comprising, in order from the bottom up, an electrically conducting substrate, a photosensitizing layer of vitreous selenium-tellurium alloy, an organic top layer comprising a member selected from the group consisting of polyvinyl carbazole and a derivative thereof. This known electrophotographic plate is advantageously flexible, panchromatic and highly charge-retaining. Further, this known plate can be either highly sensitive to visible light (one advantage) or the plate can have low residual potential (one advantage). However, this known plate cannot have both of these advantages at the same time.
Accordingly, it is an object of this invention to provide an electrophotographic plate having all the advantages of the known electrophotographic plate, high sensitivity to visible light and low residual potential at the same time.
This object is achieved by making the photosensitizing layer comprising selenium and tellurium have a two-part structure composed of a first layer part 0.05 to 2 microns in thickness having 60 to 90 weight % of tellurium and a second layer part 0.05 to 1 micron in thickness having a higher concentration of selenium than that of the first layer part the second layer part being positioned between the first layer part of the photosensitizing layer and the top organic layer. This second layer part of the photosensitizing layer can also be composed of selenium alone.
This and other objects and features of this invention will become apparent upon consideration of the following detailed description taken together with the accompanying drawing, wherein: